Club career

Early career

Antonini started his career at Milan, originally as a winger, playing for 11 years in their youth teams. At the start of the 2001–02 season, he was loaned out to Serie C2 side Prato, where he made his professional debut, totalling 26 appearances and scoring 3 goals. The following year, he made 17 appearances and scored one goal in his first Serie B season, during a loan spell at Ancona. The team got promoted to Serie A and Antonini's contribution was significant, as he became a regular in the second half of the season.

Sampdoria (2003–2004)

In June 2003 he moved to Sampdoria in a co-ownership deal with the Rossoneri, for €2 million. In exchange, Ikechukwu Kalu moved to Milan in co-ownership deal for €1 million. He did not get much playing time, as he served mainly as a backup winger for Gasbarroni. However, he made his Serie A debut on 14 December, in an away match against Perugia.